US3493366A - Alloy for service in petrochemical and hydrocarbon processing - Google Patents

Description

United States atent US. Cl. 75-122 Claims ABSTRACT OF THE DISCLOSURE Nickel-chromium alloy containing especially controlled proportions of cobalt, tungsten, niobium, aluminum, titanium and optional elements is characterized at high elevated temperatures, such as 800 C. to 1000 C., by combination of properties including high strength and prolonged resistance to embrittlement, carburization and corrosion and is particularly useful for reformer tubes and other fired pressure vessels.

The present invention relates to heat resistant alloys and, more particularly, to hot workable nickel-chromiumcobalt-iron alloys suitable for long-time use at elevated temperatures.

A process of great importance in the petrochemical and hydrocarbon processing industries is the reforming of hydrocarbons by reaction with steam in the presence of a catalyst to form a mixture of hydrogen and carbon monoxide. This gas mixture is widely used under the name of synthesis gas for the synthesis of alcohol and also as a source of hydrogen for ammonia production. The reforming reaction is carried out at high temperatures, usually in the range of 800 C. to 1000 C., under elevated pressure, which may be 400 pounds per square inch (p.s.i.), in tubular reactors known as reformer tubes. These reactors are of substantial size, being commonly up to 40 feet long, from 4 inches to 6 inches in diameter and about 0.5 inch wall thickness, and are heated externally by the combustion of hydrocarbon fuels.

Providing a good material for reformer tubes presents a difiicult problem. In order to be satisfactory, a reformer tube alloy must have good tensile properties, in particular high tensile proof stress at the operating temperature, and also have good resistance to creep under stress. It must not embrittle on prolonged heating in the range of 800 C. to 1000 C.; it must resist carburization; and it must also be resistant to corrosion by the combustion products of impure fuels. Owing to the large quantities of metal required for reformer tubes, it is also important that an alloy for such use be capable of being produced in required form at low expense; that is to say, it should have a low ratio of cost to strength.

Hitherto reformer tubes have frequently been made by centrifugal casting from chromium-20% nickel steels known by the Alloy Casting Institute Designation HK. Although such cast steel tubes have adequate stress-rupture strength for some purposes, they have some undesirable brittle characteristics in the as-cast condition and sometimes become even more brittle during elevated temperature service. They also tend to be porous. A further disadvantage is that the centrifugal casting process produces a radial columnar grain structure and carburization attack can proceed along the grain boundaries and lead to catas rophic failure. There is accordingly a need for a new allow having an improved combination of properties required for reformer tube service and which is hot-workable and weldable so that it can be formed into wrought reformer tubes and other plant and apparatus that are subjected in use to similar conditions of stress, temperature and corrosive attack, including carburization.

Although many attempts have been made to overcome the foregoing difficulties and other difficulties and disadvantages, none, as far as we are aware, was entirely successful when carried into practice commercially on an industrial scale.

It has now been discovered that a special combination of alloying elements in new proportions enables providing a nickel-chromium alloy having improved characteristics of strength, toughness, resistance to carburization and other characteristics needed for use in hydrocarbon and petrochemical processing at high elevated temperatures of the order of 800 C. to 1000 C.

It is an object of the present invention to provide a new nickel-chromium alloy.

Another object of the present invention is to provide a hot workable, weldable nickelchromium-cobalt iron alloy having characteristics particularly well suited for use of the alloy under conditions of exposure to carburizing atmospheres at elevated temperatures for extended periods of time.

The invention further provides a tubular reactor for use in reforming hydrocarbons under elevated temperature and pressure conditions requiring the reactor to be characterized by high elevated temperature strength, resistance to embrittlement and resistance to carburization.

Other objects and advantages will become apparent from the following description.

Generally speaking, the present invention contemplates a hot workable, weldable nickel-chromium alloy containing carbon in an amount up to 0.25%, e.g. 0.05% to 0.25%, about 19% to about 30% chromium, about 30% to about 50% nickel and about 2% to about 16% cobalt with the sum of the nickel and cobalt contents being at least 40%, 3% to 10% tungsten, 0.5% to 4% niobium, 0.15% to 0.6% aluminum, 0.15% to 0.6% titanium, up to about 4% molybdenum, up to 0.75% silicon, up to about 1% manganese with the balance, apart fro-m impurities, being essentially iron in amounts up to 37%. In addition, small amounts of other elements for purposes such as deoxidation malleabilization, etc., e.g., up to 0.1% magnesium, can also be included in the alloy. All alloy composition percentages set forth herein are by weight.

It is important that each of the elements mentioned is present within the ranges specified. Carbon contributes to the tensile strength, and at least 0.05% is preferably present, but at contents above 0.25% the impact strength falls off. Advantageously, the carbon content does not exceed 0.1%. Chromium contributes to the resistance of the allOy to oxidation, and at least 19% is required. At chromium contents above 30% the alloys tend to embrittle on prolonged exposure to high temperatures. Beneficially, the chromium content does not exceed 25%.

Cobalt improves the stress-rupture life, impact-resistance and corrosion-resistance of the alloy. At least 2% must be present, and advantageously the cobalt content is from 2% to 10%. A total nickel and cobalt content of at least 40% is also necessary to avoid embrittlement on prolonged heating.

Tungsten and molybdenum make an important contribution to stress-rupture life and tensile strength at elevated temperatures. At least 3% tungsten is required for this purpose, but if the tungsten content exceeds 10 embrittlement occurs. Preferably, the tungsten content does not exceed 6% when the total of the nickel and cobalt contents is less than 48%, and does not exceed 8% when the total nickel and cobalt content is 48% or more. Molybdenum supplements the effects of the tungsten at service temperatures up to 800 C. Above this temperature, the presence of molybdenum promotes excessive oxidation and for service at higher temperatures, e.g., 900 C. to 1000 C., the alloys are advantageously molybdenum-free.

The presence of small amounts of niobium in combination with the controlled amounts of other elements in the alloy has been found to be beneficial to the resistance to carburization and green rot, and at least 0.5% niobium must be present. Amounts of niobium greater than 4% lead to embrittlement and advantageously the niobium content does not exceed 2%.

As those skilled in the art will be aware, commercially available sources Of niobium, e.g. nickel-niobium alloys, commonly contain small proportions of tantalum, up to about one-tenth of the nominal niobium content. Such amounts of tantalum present as an impurity in the alloys of the invention are not disadvantageous and are to be regarded as part of the niobium content.

At least 0.15% each of titanium and aluminium is required for adequate deoxidation. If more than 0.6% of aluminium or titanium is present, carbides and intermetallic phases tend to precipitate during service, leading to weakening at the grain boundaries. While silicon can be added as a deoxidant for the alloy, the amount of silicon present must not exceed 0.75% since larger amounts render the alloys susceptible to embrittlement and impair their weldability.

For obtaining an especially advantageous combination of characteristics needed in production and use of hydrocarbon reforming reactors, the invention particularly provides a closely controlled alloy composition containing carbon up to 0.1%, 19% to 22% chromium, 38.5% to 41.5% nickel, 7% to 9% cobalt, 4% to 6% tungsten, 0.5% to 1.5% niobium, 0.2% to 0.6% titanium, 0.2% to 0.4% aluminum, 0.6% to 0.9% manganese, up to 065% silicon, up to 0.07% magnesium and the balance essentially iron.

For the purpose of giving those skilled in the art a better understanding of the invention and/or a better appreciation of the advantages of the invention, the following illustrative examples are given:

An alloy (alloy 1) which, in accordance with the invention, contained 0.09% carbon, 20.55% chromium, 39.2% nickel, 8.0% cobalt, 4.33% tungsten, 0.89% niobium, 0.50% titanium, 0.38% aluminum, 0.34% silicon, 0.83% manganese, 0.036% magnesium and the balance essentially iron was prepared by air melting in a TABLE I Test temper- Stress Life Elongation ture, C. {p.si.) (hours) (percent) TABLE II 0.1% 0.297. Ultimate Test proof proof tensile Elonga- Reduction Temp, stress stress stress tion of area C. (t.s.i.) (t.s.i.) (t.s.i.) (percent) (percent) t.s.i.=Long tons (2,240 pounds) per square inch.

TABLE III Impact strength at room temperature,

44.6 foot-pounds. 33.5 foot-pounds. 38.0 foot-pounds.

Heat treatment condition The characteristics of high impact strength and of resistance to embrittlement possessed by the new alloy, as illustrated by the test results in Table III, are a marked improvement, of greater order of magnitude, as compared with the as-cast impact strength, by a similar test, of 1.5 footpounds for centrifugally-cast 25% chromium20% nickel steel, which strength fell to 1.2 foot-pounds after heating at 800 C. for 1000 hours.

In addition to possessing very good mechanical characteristics, as illustrated in the foregoing tables, the new alloy also has very good resistance to carburization during exposure to reducing carbonaceous gases for prolonged periods of time, as illustrated hereinafter by test results pertaining to alloy 2. The chemical composition of alloy 2, which is in accordance with the invention, is set forth in Table IV and carburizing test results pertaining to alloy 2 are set forth in Table V. In order to further illustrate advantages of the invention, chemical compositions and test results pertaining to two other alloys (alloys A and B), which do not contain niobium and are not in accordance with the invention, are also included in Tables IV and V.

TABLE IV C Cr Ni Co W Mo Nb Al Ti Percent Percent Percent Percent Percent Percent Percent Percent Percent Fe 0. 08 20 36 8 6 1 0. 35 0. 37 Ba]. 0. 08 20 36 8 6 1 0. 32 0. 42 Bal. 0. 08 20 36. 8 6 0. 36 0. 40 Eat.

Ba1.=balance (including small amounts of silicon up to 0.4%, manganese up to 0.8% and magnesium up to 0.05%)

Alloys having compositions set forth in Table IV were air-melted, cast as ingots and then forged down to 0.75 inch diameter bar. Specimens taken from bars of alloys 2, A and B were exposed to a mixture of carbon dioxide with 40% by volume of carbon monoxide in a horizontal tube furnace while being maintained at a temperature of 900 C. for prolonged periods of 700 hours and 1000 hours. After being thus exposed to a carburizing atmosphere, the samples were descaled and the weight loss was determined. Weight losses which were undergone by the samples in the foregoing tests are set forth in Table V hereinafter. The test of alloy A was discontinued after 700 hours.

TABLE V Weight loss in milligrams per square centimeter After 700 hrs. After 1,000 hrs.

exposure exposure As illustrated in Table V, the alloy of the invention has superior resistance to carburization. Moreover, it is to be especially noted that comparison of the weight losses after 700 hours and 1000 hours for alloys 2 and B indicates that the rate of weight loss of alloy B was progressing much more rapidly than the weight loss of alloy 2.

As pointed out hereinbefore, cobalt is an essential constituent of the alloys according to the invention, and is not equivalent to nickel. The improvement brought about by the replacement of nickel by cobalt may be illustrated by comparison of the properties of an alloy containing 40% nickel and 8% cobalt with those of an otherwise identical alloy containing 48% nickel and no cobalt. The stress-rupture life of the latter alloy at 900 C. was not more than one-fifth of that of the former alloy and its impact strength in the annealed condition was less than half of that of the former alloy.

The alloys can be readily welded and have good resistance to oxidation and corrosion. Apart from annealing to relieve stresses they do not require heat treatment to develop their strength properties.

The alloy of the persent invention is particularly applicable for the production of wrought products, including tubing, rod, plate, sheet, forgings, etc. Wrought products of the new alloy can be used for heat and corrosion resistant articles and structures, such as wrought, welded or unwelded, reactor vessels, e.g., reformer tubes and supcrheater tubes for boilers, and for other petrochemical and hydrocarbon processing apparatus which must endure exposure to heat, stress and corrosive attack for extended periods of time. As indicated hereinbefore, special embodiments of the alloy which are essentially devoid (free) of molybdenum, e.g., contain not more than 0.15 molybdenum, are advantageous for use at temperatures exceed ing 800 C. and up to 1100 C., e.g., 900 C. to 1100 C.

Reactor vessel made of the alloy provided herein have utility particularly in the petrochemical industry for the catalystic conversion of hydrocarbons to carbon monoxide and are generally useful in other gas plant apparatus, including fired pressure vessels.

We claim:

1. A hot workable alloy consisting essentially of carbon up to 0.25%, about 19% to about 30% chromium, about 30% to about 50% nickel and about 2% to about 16% cobalt with the sum of the nickel and cobalt contents being at least 40%, 3% to 10% tungsten, 0.5% to 2% niobium, 0.15% to 0.6% aluminum, 0.15% to 0.6% titanium, up to about 4% molybdenum, up to 0.75% silicon, up to about 1% manganese, up to about 0.1% magnesium with the balance iron in an amount up to about 37%.

2. An alloy as set forth in claim 1 wherein the carbon content is at least 0.05%.

3. An alloy as set forth in claim 1 wherein the cobalt content does not exceed 10%.

4. An alloy as set forth in claim 1 wherein the carbon content does not exceed 0.1%.

5. An alloy as set forth in claim 1 wherein the alloy is essentially devoid of molybdenum.

6. An alloy as set forth in claim 1 containing carbon up to 0.1%, 19% to 22% chromium, 38.5% to 41.5% nickel, 7% to 9% cobalt, 4% to 6% tungsten, 0.5% to 1.5% niobium, 0.2% to 0.6% titanium, 0.2% to 0.4% aluminum, 0.6% to 0.9% manganese, up to 0.65% silicon, up to 0.07% magnesium and the balance iron.

7. An alloy as set forth in claim 6 wherein the carbon content is at least 0.05%.

8. An alloy as set forth in claim 1 wherein the tungsten content does not exceed 8% and does not exceed 6% when the total of nickel plus cobalts is less than 48%.

9. A tubular gas reactor vessel subjected in use to stress and corrosive attack at elevated temperatures of 800 C. and higher made of the alloy set forth in claim 1 and characterized by high strength and resistance to impact and carburization during prolonged elevated-temperature exposure for periods of at least 1000 hours at elevated temperatures of at least 800 C.

10. An alloy as set forth in claim 1 containing 4% to 6% tungsten and 0.5% to 1.5% niobium and essentially devoid of molybdenum.

References Cited UNITED STATES PATENTS 2,247,643 7/1941 Rohn et al. 171 2,403,128 7/1946 Scott et al 75171 2,553,330 5/1951 Post et a1. 75122 2,763,543 9/1956 Wagner 75122 2,873,187 2/1959 Dyrkacz et al 75124 3,046,108 7/1962 Eiselstein 75171 CHARLES N. LOVELL, Primary Examiner US. Cl. X.R.